1. Field of the Invention
The present invention relates to printed circuit boards, radio wave receiving converters and antenna devices. More particularly, the present invention relates to a structure of printed circuit board suffering less bowing after heat treatment for reflow or the like, and a radio wave receiving converter and an antenna device each provided with the relevant printed circuit board.
2. Description of the Background Art
Conventionally, a printed circuit board having a wiring pattern formed on a substrate comprised of an insulator is widely known. An example of such a printed circuit board is a high-frequency circuit board. A microstrip line and a coplanar line are often used as lines formed on the high-frequency circuit board.
A substrate portion on which, e.g., a microstrip line is formed (hereinafter, referred to as the “microstrip substrate”) consists of a component surface (the surface on which a transmission line is formed) and a ground surface.
FIGS. 13 and 14 schematically show the microstrip substrate. As shown in FIGS. 13 and 14, the microstrip substrate has a substrate 2 of a dielectric material, a strip line 1 formed on a front surface of substrate 2, and a ground pattern 3 formed on a rear surface of substrate 2. Ground pattern 3 is generally formed of a metal layer covering the entire ground surface of the microstrip substrate. Herein, such a metal layer covering the entire surface is also referred to as a pattern. By comparison, a coplanar circuit generally has a ground pattern surrounding the periphery of a transmission line.
For example, when the microstrip substrate as described above is subjected to heat treatment for reflow or the like, substrate 2 suffers bowing due to a difference in area ratio (hereinafter, referred to as the “residual copper ratio”) of the metal left on the front surface and the rear surface of substrate 2. The bowing of substrate 2 becomes greater as the difference in residual copper ratio between the front and rear surfaces of substrate 2 increases.
Generally, such bowing due to heat treatment is unlikely to occur with a hard substrate. On the contrary, a substrate formed of polytetrafluoroethylene and a soft substrate such as a so-called flexible substrate suffer considerable bowing, which greatly affects performance and quality of the circuit board.
Thus, various countermeasures have conventionally be taken to prevent the bowing, which include massaging the substrate before reflow, performing reflow at low temperature, and providing an additional step of calibrating the bowing.
Such countermeasures, however, cannot prevent bowing of the circuit board completely, causing falling off or misalignment of component(s) during reflow. Further, chip crack would occur when mounting the circuit board in an enclosure, possibly causing a serious problem regarding quality. Still further, the additional steps of massaging the substrate and others would lead to a considerable loss.